1. Field of the Invention
The present invention relates to an image forming apparatus that is suitably usable for a copying machine, a printer, a plotter, a facsimile machine, or the like.
2. Description of Related Art
One image forming apparatus that has been conventionally known is disclosed in U.S. Pat. No. 3,689,935 in which an electrode having plural opening portions (hereinafter referred to as "apertures") is used, and a voltage is applied to the electrode in accordance with image data to control passage of toner particles through the apertures, whereby an image is formed on a supporter (image receiving medium) with toner particles that have past through the apertures.
The image forming apparatus includes an aperture electrode unit comprising an insulating flat plate, a reference electrode formed continuously on one surface of the flat plate, plural control electrodes that are formed on the other surface of the flat plate and that are electrically insulated from one another, and at least one row of apertures each of which is formed in correspondence with each control electrode so as to penetrate through the flat plate, the reference electrode and the control electrodes. The image forming apparatus also includes structure for selectively applying a voltage across the reference electrode and the control electrodes, structure for supplying charged toner particles so that the flow of the toner particles passed through the apertures is modulated in accordance with the applied voltage, and structure for moving a supporter and the aperture electrode unit relative to each other to position the supporter in a particle flow passage.
U.S. Pat. Nos. 4,743,926, 4,755,837, 4,780,733, and 4,814,796 disclose image forming devices having an aperture electrode unit disposed so that control electrodes face a supporter and a reference electrode faces a toner supply side.
On the other hand, U.S. Pat. No. 4,912,489 discloses an aperture electrode unit disposed so that the reference electrode faces the supporter and the control electrodes face the toner supply side. The reference describes that a voltage to be applied to the control electrodes at an off-time can be reduced to about a quarter of that of the image forming apparatus as disclosed in the above patents.
The term "off-time" means a time when no toner particle is attached onto the supporter, that is, when a blank portion of an image is formed on the supporter, and conversely, the term "on-time" means a time when a toner image is formed on the supporter.
However, in the conventional image forming apparatus, it has been difficult to obtain a sufficient recording speed. In addition, it has been found that toner is not sufficiently supplied beneath the apertures of the aperture electrode unit. That is, since the amount of toner that is supplied to the aperture electrode unit is not sufficiently supplied to the supporter, which is fed at a high speed, a print operation with high toner density cannot be performed even with a highly efficient electric-field control.
Still further, when the apertures are arranged on a row in a width direction of the aperture electrode unit, some of the supplied toner particles that are just below the apertures are past over the aperture electrode unit without being used, and these toner particles do not contribute to the image formation in the print operation.